Currently there are approximately 60,000 lumbar spine fusions performed in the United States and 30,000 lumbar fusions performed in Canada each year. Spinal fusion is frequently used as a treatment for low back pain and intervertebral disk degeneration, and the use of internal fixation has increased the ability of a surgeon to obtain a solid fusion. There is increased concern, however, that the biomechanical rigidity of internal fixation may predispose adjacent spinal motion segments to rapid deterioration. Long-term follow up of patients undergoing a successful fusion indicates that 50 percent will continue to have complaints of pain. As in other joints, alternatives to fusing a spinal motion segment have inherent advantages.
Researchers have attempted to design a successful intervertebral disk arthroplasty for years. U.S. Pat. No. 4,946,378 discloses an artificial disk having a pair of end bodies with a medical synthetic polymeric intermediate member held between the end bodies. The intermediate member apparently provides some flexibility. Somewhat similarly, U.S. Pat. No. 5,002,576 discloses an artificial disk having end cover plates separated by a closed corrugated tube which is filled with a visco elastic material, like a body-compatible silicone.
Other approaches are shown in U.S. Pat. Nos. 4,759,769 and 4,997,432. U.S. Pat. No. 4,759,769 discloses an artificial disk having upper and lower members hinged together at a rear portion and biased apart at a front portion by stiff coil springs. U.S. Pat. No. 4,997,432 shows an artificial disk having plates separated by a sliding core body normally consisting of a synthetic material.
Although prosthetic disks continue in development, none is yet recognized as solving the disk replacement problem. There are certain basic criteria a successful intervertebral disk arthroplasty must fulfill. Fatigue strength of the materials is of utmost importance. Since the average age of patients undergoing spinal fusion is 42 years old, the life span of the device should exceed 40 years. Assuming the average person experiences 2 million strides per year and 125,000 significant bends in the spine, a conservative estimate of the number of spinal loading cycles over the 40-year period would be 85 million cycles. To provide a factor of safety, the device should be designed to at least a fatigue limit of 100 million cycles.
In addition to such durability, the materials for a successful intervertebral disk arthroplasty must be biocompatible. The volume of wear must be kept to a minimum. Although the implant should be small enough to be contained within the anatomic confines of a normal disk space, it is recognized that it may be advantageous to increase the prosthetic disk height in order to over distract the disk space to unload the facet joints posteriorly. Projections from this space may be used to provide short-term fixation, but there should then also be provision for disks to be implanted at contiguous spaces with no overlap of projections.
The present invention not only satisfies these criteria, but it is anticipated that it could be a successful arthroplasty in place of 90 percent of the fusions currently being performed.